Geological Carbon Sequestration: new insights from in-situ Synchrotron X-ray Microtomography

In a world with rapidly increasing atmospheric CO₂ concentrations, a variety of scalable technologies are being considered to mitigate emissions from the combustion of fossil fuels; among these approaches, geological carbon storage (GCS) is being actively tested at a variety of subsurface sites. Despite these activities, a mechanistic understanding of multiphase flow in scCO₂/brine systems at the pore scale is still being developed. The distribution of scCO₂ in the pore space controls a variety of processes at the continuum scale including CO₂ dissolution rate (by way of brine/CO₂ contact area), capillary trapping, and residual brine fraction. Virtually no dynamic measurements of the pore-scale distribution of scCO₂ in real geological samples have been made in three dimensions leaving models describing multi-phase fluid dynamics, reactive transport, and geophysical properties reliant on analog systems (often using fewer spatial dimensions, different fluids, or lower pressures) or theoretical models describing phase configurations. We present dynamic pore-scale imagery of scCO₂ invasion dynamics in a 3D geological sample, in this case a quartz-rich sandstone core extracted from the Domengine Fm, a regionally extensive unit which is currently a target for future GCS operations in the Sacramento Basin. This dataset, acquired using synchrotron X-ray micro tomography (SXR-μCT) and high speed radiography, was made possible by development of a controlled P/T flow-through triaxial cell compatible with X-ray imaging in the 8-40 keV range. These experiments successfully resolved scCO2 and brine phases at a spatial resolution of 4.47 μm while the sample was kept at in situ conditions (45°C, 9 MPa pore pressure, 14 MPa hydrostatic confining stress) during drainage and imbibition cycles. Image volumes of the dry, brine saturated, and partially scCO₂ saturated sample were captured and were used to correlate aspects of rock microstructure to development of the invasion front. State-of-the-art analytical techniques were applied to the multiple datasets to characterize in a quantitative fashion both the statistics of the sand grains and of the pore space. This analysis included size distribution of the sand grains in different levels of the sample, their surface area, shape parameters, and preferred orientation. The pore space was characterized in a similar fashion with captured attributes, including local porosity, pore throat network characteristics, anisotropy, and local pore diameter. Results showed a clear relationship between pore space microstructure and the scCO₂ invasion process: scCO₂ was unable to displace the brine in the parts of the sample where pores show the smaller local pore diameter values, while the invasion where the pores are larger appears to be almost complete. This result is consistent with invasion (i.e. drainage) at low capillary numbers (Ca ~ 10^-7). The zones of smaller mean pore throat size are spatially correlated in several small areas and appear to be micro-laminations generated during sediment deposition. A relevant question is what fraction of flow heterogeneity in GCS systems is driven by similar small scale variations in fabric. The results obtained show how in-situ SXR-μCT experiments are capable of providing significant information about scCO₂ flow processes in samples of sandstones relevant to GCS at the μm->cm scale.